1. Field of the Disclosure
The present disclosure relates to a method of locating seismic features in an earth formation.
2. Description of the Related Art
Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface impedance contrasts. The obtained image often suffers in spatial accuracy, resolution, and coherence due to the long travel paths between source, reflector, and receiver. In particular, due to the two-way passage of seismic signals through a highly absorptive near surface weathered layer with a low, laterally varying velocity, subsurface images are poor quality. To overcome this difficulty, a technique commonly known as vertical seismic profiling (VSP) was developed to image the subsurface in the vicinity of a borehole. With VSP, a surface seismic source is used and signals are received at a single downhole receiver or an array of downhole receivers. This is repeated for different depths of the receiver (or receiver array). In offset VSP, a plurality of spaced apart sources may be sequentially activated, enabling imaging of a larger range of distances than would be possible with a single source.
VSP measurements made during drilling operations are referred to as Seismic-while-drilling (SWD®). The signals generated by seismic sources are reproducible and may be stacked. The other kind of data recorded by the seismic sensors is noise. Background noise (drilling noise, circulation noise, rig noise, cultural noise, environmental noise) may be distinguished from spiky noise (e.g. due to hitting the drill string while connection; micro earthquakes close to the borehole). It may be helpful to restrict seismic recording of data to low-noise periods. It may also helpful to stack the data. Because the noise is random, stacking the data may increase the signal to noise ratio.
The SWD® measurements may include the VSP-while drilling (VSP-WD) method and the checkshot-WD (CS-WD) method. These methods permit updating of the geological model. In this way one can reduce the drilling risk and/or update the optimal well path. The importance of real-time processing is evident, but a full real-time processing isn't possible yet. One limitation of Seismic-while-drilling measurements is the small bandwidth of the uplinks and downlinks. The communication is done via mud telemetry, which is possible only while circulating. Hence when tripping in or tripping out, measurements are done without circulating between shooting windows, and it is not possible to send uplink signals and downlink signals. Even when mud telemetry is possible, the bandwidth available for uplink signals and downlink signals is very small.
Due to the small uplink bandwidth, the downhole tool must automatically detect and process the shooting sequence(s) downhole. Only the final results (e.g., the first-break time) are sent to the surface.
Another limitation VSP-WD is the requirement highly accurate clocks (on the order of 1 millisecond time drift per ten day period) that are synchronized for performing measurements. The requirements for high accuracy and synchronization may result in high complexity and cost.
There is a need for a method of performing seismic measurements that may use less accurate, and hence less complex and less expensive, clocks and synchronization systems. The present disclosure addresses this need.